The invention relates generally to downhole motors. More particularly, the invention relates to coupling devices for converting the eccentric rotation of the rotor of a downhole motor into concentric rotation.
In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a “drill string”, then rotate the drill bit while applying weight-on-bit to allow the bit to progress downward into the earth along a predetermined path to form a borehole. A typical drill string is made up from an assembly of drill pipe sections connected end-to-end, plus a “bottom hole assembly” (BHA) disposed between the bottom of the drill pipe sections and the drill bit. The BHA is typically made up of sub-components such as drill collars, stabilizers, reamers and/or other drilling tools and accessories, selected to suit the particular requirements of the well being drilled.
The drill string and bit are often rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel). During the drilling process, a drilling fluid (commonly referred to as “drilling mud” or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the annular space (“wellbore annulus”) between the drill string and the wellbore. The drilling fluid carries borehole cuttings to the surface, cools the drill bit, and forms a protective cake on the borehole wall (to stabilize and seal the borehole wall), as well as other beneficial functions. At the surface the drilling fluid is treated, by removing borehole cuttings, amongst other possible treatments, then re-circulated by pumping it downhole under pressure through the drill string.
As an alternative to rotation by a rotary table or top drive alone, a drill bit can also be rotated using a “downhole motor” incorporated into the drill string immediately above the drill bit. The technique of drilling by rotating the drill bit with a downhole motor without rotating the drill string is commonly referred to as “slide” drilling. It is common in certain types of well-drilling operations to use both slide drilling and drill string rotation, at different stages of the operation. The use of downhole motors has generally increased in recent years due, at least in part, to their employment in the drilling of wellbores directionally, since downhole motors provide some advantages in such applications.
The downhole motor, which may also be referred to as a mud motor or progressive displacement motor (PDM), converts hydraulic energy of a fluid such as drilling mud into mechanical energy in the form of rotational speed and torque output, which may be harnessed for a variety of applications such as downhole drilling. A typical downhole motor includes a hydraulic drive section, a drive shaft assembly, and a bearing assembly. The hydraulic drive section, also known as a power section or rotor-stator assembly, includes a helical rotor rotatably disposed within a stator, the driveshaft assembly includes a driveshaft rotatably disposed within a driveshaft housing, and the bearing assembly includes a mandrel rotatably supported within a housing. The lower end of the rotor is connected to the upper end of the driveshaft, the lower end of the driveshaft is connected to the upper end of the mandrel, and the lower end of the mandrel is coupled to a drill bit. During drilling operations, the high pressure drilling fluid is pumped under pressure down the drillstring and between the rotor and stator, causing the rotor to rotate relative to the stator. Rotation of the rotor drives the rotation of the driveshaft, the mandrel, and the drill bit.
The central axis of the rotor is typically radially offset from the central axis of the stator by a fixed value known as the “eccentricity.” As a result, the rotor rotates eccentrically within the stator. However, many components of the BHA and drill string which utilize the torque generated by the downhole motor (e.g., power generators, drill bit, etc.) are aligned with the central axis of the drill string and stator. Thus, to utilize the torque supplied by the downhole motor the eccentric motion of the rotor is converted to concentric rotation via the driveshaft.